Special Regulation for Paternal Transmission of Mitochondrial DNA May Exist in Cucumber

Mitochondria, the remnant of their ancestral bacteria-endosymbiont genome maintains its own genome in plant cells. Reportedly mitochondrial (mt) genome is maternally inherited in the most angiosperms. But cucumber (Cucumis sativus L.) is an exception with paternal transmission of the mtDNA. Although this phenomenon was observed more than 10 years ago, the mechanism is still unknown. Previous studies indicated that the mode of mtDNA inheritance could be classified according to the quantity of mtDNA in male gametes. In the case of paternal transmission, male gametes possess higher level of mtDNA compared to mesophyll cells. However, our preliminary experiments found that in cucumber each male gamete possessed less mtDNA, different from C. melo which is also characterized with paternal transmission of mtDNA and possessed markedly higher levels of mtDNA. To understand this unique reduction in cucumber, cell DAPI staining and quantitative PCR were used to quantify the copy numbers of mtDNA at different pollen development stages. In addition to the two characteristic signals corresponding to the vegetative nucleus and the generative nucleus, many smaller but intense DAPI signals were seen around the generative nucleus. On average, while 56.3 copies of mtDNA were detected in the mesophyll cells, 208.5, 256.6, and 20.8 copies of mtDNA were detected with qPCR at late uninucleate, rounded generative nucleus, and bicellular stage, respectively, showing the fluctuation of mitochondrial copy number during pollen development. These results suggest that a special regulation may exist with regard to the paternal transmission of mtDNA in cucumber.